Revealing Mechanism and Origin of Reactivity of Au(I)-Catalyzed Functionalized Indenone Formation of Cyclic and Acyclic Acetals of Alkynylaldehydes

Density functional theory exploited with the (SMD)-B3LYP-D3/def2-TZVP//B3LYP/6-31G(d),LANL2DZ level of theory is presented to offer mechanistic insights and explications of experimentally intriguing observations in the Au(I)-catalyzed cyclization of cyclic and acyclic acetals of alkynylaldehydes that lead to indenone formation. The reactivity of catalytic cycles with and without methoxy migration is computationally defined when alkyne terminus is phenylated in addition to the unreactive cycle when alkyne terminus is not phenylated. The reaction mechanism of indenone formation proceeds first with coordination of Au(I) to alkyne to initiate the reaction with 1,5-H shift as a rate-determining step and the fastest 1,5-H shift is achieved when one phenyl ring carries electron-donating group and the other one is substituted with electron-withdrawing group. The absence of tethered acetal unit considerably outpaces any 1,5-H shift and instead activates 1,5-methoxy migration, giving methoxy-migrated indenone, with the step of 1,2-OMe shift is a rate-limiting step during reaction pathway. Following 1,5-H shift the cyclization and 1,2-H shift are kinetically and thermodynamically feasible, which are followed by elimination to persist the iterative cycle, but the reactivity of both steps is highly affected by the existence of phenyl group on alkyne terminus. The unreactivity of alkyne terminus being not beared a phenyl ring is due to that the cyclization is thermodynamically disfavorable, subsequently deactivating the 1,2-H shift kinetically and thermodynamically.